Dave Hubbard showed us how to turn grease from restaurants into fuel for his black Jetta TDI, a BMW motorcycle, bulldozers, tractors for five local farms and a tree nursery—all for the low cost of about 50 cents a gallon. Now this engineer and home-grown chemist has a warning for those who may try to follow in his footsteps: Watch what vehicle you buy. Pure biodiesel, once compatible with all diesel engines, now leads to problems in new cars.

I have been experimenting with making biodiesel for a few years now, and I am constantly watching what others in this field do as well. I monitor a forum for biodiesel users such as myself, and it's full of practical advice as well as troubleshooting for the problems that sometimes arise. Biodiesel appeals to us because it can be made from plants grown locally and burns far cleaner then ordinary diesel fuel. It results in 67 percent less unburned hydrocarbons (helping to reduce smog and ozone), 48 percent less carbon monoxide and 47 percent less particulate matter, according to an analysis of heavy-duty engines by the Environmental Protection Agency. Only nitrogen oxides, or NOX, slightly increase. It can also be blended with regular diesel in any percentage from B2 (98 percent diesel, 2 percent biodiesel) to B100, which is pure biodiesel. One of the biggest problems we biodiesel makers have experienced recently has nothing to do with the conversion process--it has to do with diesel engines.

Until two years ago, all diesel engines were B100-compatible (biodiesel cannot run in gasoline engines because it needs an engine that ignites by compression). Then standards set by both the Environment Protection Agency and California Air Resources Board, phased in for 2007, required all passenger vehicles to meet the same, stricter emissions. That meant diesel manufacturers had to reduce emissions of NOX and particulate matter to meet those of gas-powered cars. These standards were created with good intentions--to look out for our health by improving the air that we breath. (After all, particulate matter is a known carcinogen.) But the way most manufacturers did this created a setback for those of us trying to use biofuels.

To get rid of particulate matter, the diesel manufacturers came up with what's called a DPF (diesel particulate filter). But this catalytic filter becomes poisoned if sulfur dioxide is pumped through it. So as of 2007, the standard for diesel fuel was revamped as well, and fuel refineries had to reduce sulfur content to no more than 15 parts per million (now known as ultralow-sulfur diesel). The DPF is placed in the exhaust system in front of the muffler and looks like a catalytic converter used on gasoline engines. It captures particulate matter in its inner core. Periodically, the DPF has to be taken up to high temperatures to burn off the matter it has collected. This is called regeneration or postinjection regeneration. The idea is to inject fuel into the exhaust that has been vaporized, and when the fuel comes into contact with the DPF, an exothermic reaction heats it up and incinerates the plug of soot. (Squirting fuel down the exhaust? Gee, I wonder why the newer models have poorer fuel mileage.)

And here is where the pitfall lies for biodiesel users like myself. Most of the manufactures decided to inject fuel into the cylinders just after the cylinder fires and the exhaust valve opens. At this point, the fuel vaporizes and the vapors move down the exhaust to the DPF and clean it. Because biodiesel is denser than conventional diesel fuel (it has a longer hydrocarbon chain) and has a higher distillation temperature and boiling point, it does not vaporize as easily. Some of the fuel ends up adhering to the cylinder wall and runs past the rings, diluting engine oil.

There has been quite a bit of debate on biodiesel forums as to why manufacturers chose this method of injection. Most likely, the answer is cost. Manufacturers would rather utilize existing fuel-injection systems rather than tack on the potentially substantial cost of extra equipment. It is far cheaper to change software then it is to change hardware--just tell your electronic control unit (ECU) to apply an extra squirt of fuel. After all, they have invested a lot of money already, setting up high-pressure multiple precise injections for cleaner combustion.

Still, not all manufactures have gone this route. Some Caterpillar and Cummins engines have an injector that is placed in the exhaust pipe, not in the cylinder (in-stream fuel injection), making them biodiesel-compatible. However, these are large trucks that don't need to meet the same emissions standards as cars. Other engines clean exhaust by diesel exhaust fluid (DEF), an aqueous urea catalytic reduction called Blue Tech, otherwise known as Add Blue in Europe. This system is used on some large Dodges and some large Mercedes-Benz models. The problem with this system is you have to maintain a compartment with urea and they are mostly designed for NOX reduction.

What does this all mean for backyard biodiesel makers? Well, we can no longer brag that B100 will run in any diesel engine: VW, Mercedes, Nissan, Renault, Jeep, Ford Power Stroke, Ford E-series vans, Dodge Rams, Cummins 6.7 and Chevy Duramax will only tolerate small amounts. (For an example of what can happen if you push that limit, check out a biodiesel commercial seller who tried to run B100 in his new 2009 VW.) People in my community would like to see engine manufacturers and biodiesel makers work together to solve the B100 problem while still passing emissions standards--and selling affordable cars. In the meantime, we'll have to watch which vehicle we fill with our fuel.

A Part of Hearst Digital Media
Popular Mechanics participates in various affiliate marketing programs, which means we may get paid commissions on editorially chosen products purchased through our links to retailer sites.